The long-range goal of this proposal is to understand how the Epidermal Growth Factor Receptor (EGF-R) is controlled during metazoan development. Recessive loss-of-function mutations in many general components of receptor tyrosine kinase (RTK) signal transduction pathways, such as Sos, Ras, Raf, and MAPK, have been identified in previous screens for dosage-sensitive suppressors and enhancers of RTK signaling mutations. While the previous screens are useful in identifying mutations in many RTK signaling genes, they might have missed components that are present redundantly or in non-limiting amounts. Also, the previous screens generally isolated loss-of-function alleles of signaling genes, whereas the epistatic relationships among these genes has been determined primarily through the isolation and characterization of dominant mutations. Dominant mutations in genes have also been extensively used to analyze signaling pathways in the less genetically tractable vertebrate tissue culture systems. The investigators propose a novel genetic screen which will allow the isolation of dominant gain-of-function and dominant loss-of function mutations in genes involved in EGF-R and other signal transduction pathways. Genes that either inhibit or activate EGFR signaling will be identified by isolating dominant female sterile (DFS) mutations that disrupt the differentiation or proliferation of the follicle cells that surround the egg chambers. The genetic screen involves using newly developed techniques to recover mutations from mosaic animals. Preliminary genetic screens have already identified DFS mutations in two genes, Star-Kojak and Ugra-DFS. Both of these DFS mutations generate their phenotypes by down-regulating EGF-R signaling in the follicle cells that surround the oocyte. Loss-of-function alleles of Star and Ugra have phenotypes that suggest that they might be involved in the poorly understood area of ligand production and presentation. Since other signal transduction pathways' such as those that involve wingless, Notch, TGFb and hedgehog, are also involved in follicle cell development, a further advantage of the proposed screen is that follicle-cell dependent DFS mutations that affect these other pathways could also be isolated. In order to accomplish their goal of understanding EGF-R regulation. three objectives are paramount. Firstly, they will conduct additional .genetic screens to isolate dominant mutations, like Star-Kojak and Ugra-DFS, in other genes involved in EGF-R and other signal transduction pathways. Secondly, molecular analysis of Ugra will be performed to learn what it encodes and what role it plays in EGF-R signaling. Thirdly, they will perform a detailed molecular and phenotypic analysis of Star-Kojak and Star loss-of-function alleles to better understand the role of Star in EGF-R signal transduction. It is important to understand regulators of RTK signaling because Ras-Raf dependent neoplasia accounts for as many as one-fifth of all human cancers.